The present invention relates to a power supply unit for a press machine that includes a converter connected to a commercial AC power supply, and an inverter connected to a press motor, and can generate and output motor drive power for driving (rotating) the press motor.
As illustrated in FIG. 11, a motor drive power supply unit normally includes a commercial AC power supply 10P, a converter 30P, and an inverter 60P. AC power is supplied from the commercial AC power supply 10P to the converter 30P via a primary AC current path 11 (AC phase current paths 12, 13, and 14), and the converter 30P outputs DC power generated by AC-DC conversion to a DC current path 40 (positive current path 41 and negative current path 42). The inverter 60P subjects the DC power supplied from the converter 30P to DC-AC conversion (frequency/voltage adjustment). The resulting AC power is supplied to a motor 1P via a secondary AC power supply path 66 (phase drive current paths 67, 68, and 69). The motor 1P can thus be driven (rotated) in a predetermined direction at a predetermined speed and torque.
JP-A-2009-106146 discloses a motor drive device (power supply unit) that is developed from the above power supply unit, wherein a capacitor 8 (indicated by dot-dash line in FIG. 11) is provided between the converter 30P and the inverter 60P. In the power supply unit disclosed in JP-A-2009-106146, the converter is formed by a rectifier circuit, and the inverter is formed by an inverter circuit. The rectifier circuit converts three-phase AC power into DC power to charge the capacitor 8. The inverter is formed by six switching elements that are connected via bridge connection, and diodes that are respectively connected in inverse parallel to the switching elements. The switching elements are ON/OFF-controlled by an inverter control circuit.
The electrical energy (DC power) stored in the capacitor 8 is subjected to DC-AC conversion during a power-running operation to generate and output AC power (motor drive power). The inverter control circuit causes the regeneration energy generated when the motor is decelerated to be returned to the capacitor 8 during a regeneration operation. Specifically, the regeneration energy generated when the motor is decelerated is stored in the capacitor 8, and used as the power-running energy when the motor is accelerated.
JP-A-2010-178584 discloses a power supply unit wherein a converter circuit is formed to implement a regeneration operation, and the regeneration energy of a motor can be returned to a commercial AC power supply. According to the power supply unit disclosed in JP-A-2010-178584, the converter circuit can be repeatedly and carefully operated and stopped corresponding to the DC voltage value. JP-A-2010-178584 states that the above configuration reduces the thermal load applied to the switching element and the like.
JP-A-2003-143838 discloses a device wherein a fuse is connected between a commercial AC power supply and a power supply unit in the same manner as in a normal electrical circuit, and the power supply unit is compulsorily stopped by cutting the fuse when the amount of AC power supplied has exceeded a preset value.
A motor drive device used for a press machine that drives a slide drive mechanism (e.g., crank mechanism) by rotating a motor to move a slide (upper die) upward and downward, and presses a workpiece using the upper die and a lower die attached to a bolster, is selected from various motor drive devices (power supply units) depending on the conditions such as the working capability and the equipment cost.
For example, a power supply unit for a press machine illustrated in FIG. 12 is configured so that the electrical capacity of a converter 30 P and an inverter 60P is set to the same value (1200 kW) as the maximum load (1200 kW) of a motor 1P, and a capacitor 51P has a small capacitance (e.g., 0.3 F). The press machine performs a press operation as illustrated in FIG. 13 (where the vertical axis indicates power (kW), voltage (V), and the slide position (mm), and the horizontal axis indicates time (t)) in which the slide moves downward from top dead center (500 mm) according to the slide motion Msd, press working starts when the slide is positioned about 200 mm above bottom dead center, and ends when the slide is positioned at bottom dead center (0 mm), and the slide then moves upward toward top dead center.
As illustrated in FIG. 13, the motor output Pmt is 1000 kW, and the input power Pss input to the power supply unit is 1070 kW during press working. The input power Pss is larger than the output power Pmt since the efficiency decreases due to mechanical loss and electrical loss. The DC voltage Vdc of the DC current path (DC bus) is about 700 V before press working, and decreases to some extent during press working. However, the DC voltage Vdc is maintained to be almost constant. This is because the ratio of the motor output capacity to the output capacity of the power supply unit is balanced (i.e., 1 (1200 (1000) kW): 1 (1200 (1070) kW)).
When designing a power supply unit for a press machine, it is necessary to carefully take account of conditions specific to the characteristics of the press machine and the press working mode. For example, the press machine is characterized in that a change in load within one cycle (one slide stroke) is significantly larger than that of other industrial machines and the like. Therefore, an improvement effective for implementing a continuous press operation and a reduction in size (i.e., product cost) has been strongly desired.
There may be a case where it is difficult to employ the ideal configuration (i.e., the ratio of the motor output capacity to the output capacity of the power supply unit is set to 1:1) illustrated in FIGS. 12 and 13 that can implement a stable press operation. Specifically, since it is necessary to stably supply the maximum power for a short time (during press working), but the amount of power required during a period other than the press-working period is small, it has been desired to reduce at least the amount of input power and the size of the power supply unit.
When pursuing a reduction in size of the power supply unit, however, the motor drive energy may be insufficient, and the power supply unit may be overloaded due to an overcurrent, so that the parts of the power supply unit may be burned out. Moreover, it may be necessary to stop the press operation even if a change in load occurs only for a short time. It is necessary to avoid cutting the fuse irrespective of the reason why an overload has occurred. This is because the protection technique of cutting the fuse is not realistic since an overcurrent may frequently occur depending on the press conditions (working conditions). The productivity significantly decreases if the press operation of the press machine has been stopped. Moreover, defective products may be obtained, and it may take time to restart the press machine when the press operation has been unintentionally stopped.
Moreover, since the technical specifications of the press machine which are taken into account when designing the power supply unit may arbitrarily be changed (e.g., when the operator arbitrarily sets (changes) the slide motion), inconvenience or disadvantage may occur suddenly during actual operation.